Thus far, techniques have been known for purifying nitrogen oxides (hereinafter referred to as “NOx”) contained in exhaust.
For example, in Patent Documents 1 and 2 and Non-patent Document 1, an exhaust purification device is shown that is provided with an oxidation catalyst and a NOx occlusion-reduction catalyst (hereinafter referred to as “LNT”) in an exhaust channel. With this exhaust purification device, NOx in the exhaust having passed through the oxidation catalyst is occluded by reacting with an alkali metal, alkaline earth metal or the like during lean operation in which the exhaust is oxygen excessive, and the NOx thus occluded is reduced during rich operation in which the oxygen concentration of the exhaust is low. With this exhaust purification device, occlusion of NOx and reduction of NOx can be performed periodically by repeating lean operation and rich operation.
In addition, in Non-patent Document 2, for example, a method is shown in which NOx is adsorbed on a catalyst during lean operation in which the exhaust is oxygen excessive, then rich operation is performed and a state in which the oxygen concentration in the exhaust is low is periodically produced, while carbon monoxide is periodically synthesized and supplied, thereby periodically reducing the NOx adsorbed during lean operation.
More specifically, in the method shown in Non-patent Document 2, first, nitrogen monoxide and nitrogen dioxide existing in the exhaust is adsorbed to the catalyst during lean operation in which the exhaust is oxygen excessive, by way of the following formulas (1) to (3).NO→NO(adsorption)  (1)2NO+O2→2NO2  (2)NO2→NO2(adsorption)  (3)
Next, rich operation is performed while carbon monoxide is synthesized. The carbon monoxide thereby synthesized produces hydrogen by way of the water-gas shift reaction shown in the following formula (4), in a situation where the oxygen partial pressure is low.CO+H2O→H2+CO2  (4)
Furthermore, ammonia is produced by reacting this hydrogen with carbon monoxide in a reducing atmosphere, and this ammonia is adsorbed to the catalyst by way of the following formula (5).5H2+2NO→2NH3(adsorption)+2H2O  (5)
With the ammonia produced by carbon monoxide according to the above as the final reducing agent, NOx in the exhaust or NOx adsorbed to the catalyst is reduced by way of the following formulas (6) to (8).4NH3+4NO+O2→4N2+6H2O  (6)2NH3+NO2+NO→2N2+3H2O  (7)8NH3+6NO2→7N2+12H2O  (8)
Alternatively, in Patent Documents 3 and 4, for example, an exhaust purification system is shown that is provided with a LNT in the exhaust channel, and is further provided upstream of this LNT with a fuel reformer that produces a reducing gas containing hydrogen and carbon monoxide by reforming hydrocarbon fuel. In this exhaust purification system in particular, a fuel reformer is used that produces a reducing gas such that hydrogen is more abundant than carbon monoxide by volume ratio. According to this system, it becomes possible to selectively reduce the NOx in the exhaust by adding reducing gas containing hydrogen from an upstream side of the LNT into the exhaust.
Here, as a method of producing reducing gas from hydrocarbon fuel, for example, a partial oxidation reaction using oxygen as an oxidant has been known, as shown in the following formula (9), for example.CnHm+½nO2→nCO+½mH2  (9)
This partial oxidation reaction is an exothermic reaction using fuel and oxygen, and the reaction progresses spontaneously. As a result, once the reaction begins, hydrogen can be continuously produced without supplying heat from outside. In addition, in such a partial oxidation reaction, in a case of fuel and oxygen coming to coexist in a high temperature state, the combustion reaction as shown in the following formula (10) also progresses.CnHm+(n+¼m)O2→nCO2+½mH2O  (10)
Moreover, the steam reforming reaction, which uses steam as an oxidation, shown in the following formula (II) has been known.CnHm+nH2O→nCO+(n+½m)H2  (11)
This steam reforming reaction is an endothermic reaction using fuel and steam, and is not a reaction that progresses spontaneously. As a result, the steam reforming reaction is an easily controlled reaction relative to the partial oxidation reaction described above. On the other hand, it is necessary to input energy such as of a heat supply from outside.
Patent Document 1: Japanese Patent No. 2586738
Patent Document 2: Japanese Patent No. 2600492
Patent Document 3: Japanese Patent No. 3642273
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2002-89240
Non-patent Document 1: “Development of NOx Storage Reduction Three-way Catalyst System,” Collective Papers of Society of Automotive Engineers of Japan, Vol. 26, No. 4, October 1995
Non-patent Document 2: “A NOx Reduction System Using Ammonia Storage-Selective Catalytic Reduction in Rich and Lean Operations,” 15 Aachener Kolloquium Fahrzeug-und Motorentechnik 2006 p. 259-270
However, there is the following problem in a case of repeating lean operation and rich operation of an engine as in the technology shown in Patent Documents 1 and 2 and Non-patent Documents 1 and 2 described above, for example.
In other words, when the exhaust air/fuel ratio is made rich in order to reduce the NOx occluded to the LNT, there is a case in that, if NOx of a great amount is contained in the exhaust flowing into the LNT, the reducing agent is mostly consumed in the reduction of NOx in the exhaust. In such a case, the NOx occluded to the LNT desorbs from the exhaust air/fuel ratio being made rich; however, it will be discharged downstream of the LNT without being reduced, and thus the NOx purification performance may decline.
Incidentally, a method of lowering the NOx emitted by recirculating a portion of the exhaust to an intake side is known. However, although the emission amount of NOx can be reduced when recirculating a great amount of exhaust, the emission amount of particulates will increase. In particular, since it is difficult to recirculate a great amount of exhaust during high-load operation in which there is a tendency for the emission amount of NOx to increase, the amount of NOx discharging from the LNT may increase along with an increase in the emission amount of NOx.
In addition, when NOx occluded to the LNT is reduced, it is necessary to make the exhaust air/fuel ratio rich. As methods for making the exhaust air/fuel ratio rich, there is a method of enriching the exhaust air/fuel ratio by performing fuel injection that does not contribute to torque (hereinafter referred to as “post injection”) and flowing uncombusted fuel to an exhaust channel (hereinafter referred to as “method by post rich”), and a method of enriching by adjusting a fuel injection (hereinafter referred to as “main injection”) amount that contributes to torque (hereinafter referred to as “method by combustion rich”).
However, since fuel is injected into the cylinders in the latter half of the combustion stroke or in the exhaust stroke of the engine in the method by post rich, a portion of the fuel adheres to the cylinder walls. In other words, in the method by post rich, not all of the fuel supplied by post injection contributes to enrichment of the exhaust air/fuel ratio. As a result, the fuel economy may deteriorate compared to the method by combustion rich. In addition, the fuel adhered to the cylinder walls will mix in its current state into the engine oil, and thus so-called oil dilution may occur.
Moreover, with the method of combustion rich, the operating conditions thereof are limited. For example, during high-load operation in which combustion becomes sharp, the combustion noise grows worse. In addition, during low-load operation such as immediately after startup of the engine or while idling, the charge efficiency to the cylinders may decline and combustion may become unstable.
Alternatively, when rich operation is performed and the exhaust air/fuel ratio is made rich, although NOx occluded to the LNT desorbs, in a case in which the NOx occlusion amount of the LNT is large, NOx will be discharged downstream of the LNT without being reduced, and thus the NOx purification performance may decline.
Consequently, it has been considered to increase the frequency of performing rich operation and reduce NOx frequently so that the NOx occlusion amount of the LNT does not become large. However, the region in which rich operation can be performed is limited depending on the operation state of the engine such as revolution speed and torque. As a result, it is difficult to reduce NOx frequently independently of the operating state of the engine.
In addition, the exhaust purification system of Patent Document 3 and 4 is different from that exemplified in Patent Document 1 and 2 described above, and specifically is a system that adds hydrogen, carbon monoxide and hydrocarbons into exhaust having an oxygen excess, irrespective of lean operation and rich operation.
However, in a case of purifying with LNT by adding a reducing agent such as hydrogen, carbon monoxide, and hydrocarbons into exhaust having an oxygen excess in such a way, the ability to purify NOx is limited to approximately 200° C. For example, when the temperature of the LNT is at least 200° C., the hydrogen and carbon monoxide thus added combust. As a result, with such a temperature, the amount of additive is deficient, leading to the reduction reaction of NOx not progressing sufficiently.
In addition, in a case of providing a fuel reformer in an exhaust channel having an exhaust amount that regularly fluctuates, as in the exhaust purification systems of Patent Documents 3 and 4, it is necessary to increase the reaction time for which the reforming catalyst of the fuel reformer and the exhaust come into contact, in order to effectively produce hydrogen in this fuel reformer. However, in order to increase the reaction time as such, it is necessary to increase the size of the reforming catalyst, which may raise cost.
In addition, in order to operate the fuel reformer in a stable state, it is necessary to maintain the reaction temperature of the reforming catalyst of this fuel reformer to be constant. However, when providing a fuel reformer in an exhaust channel for which the oxygen amount, steam amount, and temperature are always fluctuating depending on the operating state of the engine, as in the exhaust purification systems of Patent Documents 3 and 4 described above, it becomes difficult to operate the fuel reformer in a stable state. In a case in which such a fuel reformer cannot be operated stably, in a case in which NOx of a large amount is contained in the exhaust as described above, or a similar case, NOx may discharge to downstream of the LNT without being completely reduced.
Incidentally, sulfur components in the fuel and engine oil are contained in exhaust emitted from the internal combustion engine. When such sulfur components accumulate on the NOx purification catalyst, the NOx purification performance declines. Therefore, the following plurality of techniques has been proposed that aim to prevent the purification performance from declining due to such poisoning of the NOx purification catalyst.
The most general method is to execute a regeneration process of purifying sulfur components adhered to the NOx purification catalyst by making the exhaust air/fuel ratio lower than a stoichiometric ratio over a predetermined time period and making the NOx purification catalyst to be high temperature.
Herein, as a method of controlling the exhaust air/fuel ratio when performing the regeneration process, in addition to the method by combustion rich and the method by post rich described above, a method has also been known of directly injecting fuel into the exhaust channel (hereinafter referred to as “method by exhaust injection”).
Alternatively, an exhaust purification device that provides a fuel reformer, which produces a reducing gas containing hydrogen, carbon monoxide, etc. by way of a reforming reaction, upstream of a NOx purification catalyst is proposed in Patent Document 5. According to this exhaust purification device, removal of sulfur components is promoted by adding hydrogen thus produced by the fuel reformer to the exhaust when executing the regeneration process of the NOx purification catalyst.
In addition, an exhaust purification device that purifies NOx in exhaust by producing hydrogen from hydrocarbons and steam by a plasma generator and adding this hydrogen to the NOx purification catalyst, as well as that prevents sulfur component from adhering to the NOx purification catalyst by suppressing oxidation of SO2 is proposed in Patent Document 6.
Moreover, the matter of executing a regeneration process of a NOx purification catalyst efficiently and over a short time period by selectively supplying hydrogen and gasoline depending on the temperature of the NOx purification catalyst of an exhaust purification device of an engine that employs hydrogen as a fuel is proposed in Patent Document 7.
Patent Document 5: Japanese Patent No. 3896923
Patent Document 6: Japanese Unexamined Patent Application Publication No. 2004-270587
Patent Document 7: Japanese Unexamined Patent Application Publication No. 2006-307679
However, the following issues exist in the techniques as described above.
First, in the regeneration process of the NOx purification catalyst, when controlling the exhaust air/fuel ratio by the method of exhaust injection, uncombusted fuel makes direct contact with the NOx purification catalyst and other catalysts, the temperature of the catalyst surface becomes high locally under an oxidizing atmosphere, and thus the catalyst may deteriorate due to sintering or the like. In addition, when fuel contacts the catalyst in a droplet state, the temperature of the catalyst surface at the contacting portion thereof will decrease locally due to the latent heat of vaporization, and thus coking may occur. In particular, if exhaust injection is performed in a case in which the exhaust temperature is low, the exhaust temperature will decrease further due to the latent heat of vaporization of the fuel supplied by exhaust injection, whereby liquid fuel collects in the exhaust channel, the catalyst deteriorates, and exhaust system components may corrode.
In addition, in the regeneration process of the NOx purification catalyst, when controlling the exhaust air/fuel ratio by the method of post injection, a portion of the fuel injected adheres to the surface of the wall of the cylinders, and thus this fuel may mix into the engine oil. In such a case, not only does the fuel injected not contribute to the purification of the sulfur component, but also so-called oil dilution in which engine oil is diluted by this fuel may occur.
In addition, in the regeneration process of the NOx purification catalyst, in a case of controlling the exhaust air/fuel ratio by the method of combustion rich, for example, combustion may become unstable if a state in which the intake air amount must be drastically decreased such as during low-load operation is continued. As a result, in a case of transitioning to idle operation or deceleration operation, the exhaust air/fuel ratio at this time must be returned to lean.
Incidentally, the NOx purification catalyst has a function of adsorbing oxygen in exhaust when the exhaust air/fuel ratio is lean. As a result, reducing agents in the exhaust will react with oxygen adsorbed while lean immediately after the exhaust air/fuel ratio is switched from lean to rich, and thus it becomes difficult for sulfur component adhered to the NOx purification catalyst to desorb. Therefore, if the frequency at which the exhaust air/fuel ratio is returned to lean increases as described above, control for causing the sulfur to desorb becomes an extra necessity along with this, and the NOx purification catalyst may degrade and the fuel economy may deteriorate.
In a case of providing a fuel reformer in an exhaust channel as in the exhaust purification device of Patent Document 5, there are the following issues.
In other words, due to the heat capacity upstream of the NOx purification catalyst increasing by providing the fuel reformer in the exhaust channel, the time from when the engine is started up to the activation temperature of the NOx purification catalyst is reached becomes long. As a result, the NOx purification performance particularly immediately after engine startup may decrease. Consequently, although providing a heat riser in order to avoid this has also been considered, a cost may be associated therewith.
In addition, in a case of providing the fuel reformer in an exhaust channel having an exhaust amount that regularly fluctuates, it is necessary to increase the reaction time for which the reforming catalyst of the fuel reformer and the exhaust come into contact, in order to effectively produce hydrogen in this fuel reformer. However, in order to increase the reaction time as such, it is necessary to increase the size of the reforming catalyst, which may raise cost.
In addition, in order to operate the fuel reformer in a stable state, it is necessary to maintain the reaction temperature of the reforming catalyst of this fuel reformer to be constant. However, when providing a fuel reformer in an exhaust channel for which the oxygen amount, steam amount, flow-rate, and temperature are always fluctuating as described above, it becomes difficult to operate the fuel reformer in a stable state.
With the exhaust purification device of Patent Document 6, the fuel economy may deteriorate due to there being a necessity to constantly produce hydrogen by causing plasma to be generated.
In addition, with the exhaust purification device of Patent Document 7, there is a necessity to provide a plurality of fuel tanks in order to supply hydrogen and gasoline separately. As a result, the device may increase in size, the maintenance properties may decline, and control thereof may become complex.